Electric vehicles, such as fork lift trucks, transit cars, etc., generally provide for electrical braking of the traction motor to assist the mechanical brakes in bringing the vehicle to a stop. In producing an electrical braking effort, the traction motor is electrically controlled to operate as an electrical generator, driven by at least one of the rolling wheels of the vehicle. Thus, as braking is being carried out, the traction motor is effective to convert the kinetic energy of the vehicle to electrical energy.
The method of disposing of this electrical energy classifies the mode of electrical braking being utilized. Generally, electrical braking is either dynamic, in which case the electrical energy is dissipated as heat in a resistive load, or regenerative, in which case the electrical energy is transferred back to the power source. A subset of dynamic braking is "plug" braking in which the armature resistance of the motor is itself used as the resistive load.
Obviously, regenerative braking is preferred if the power source is able to accept the regenerative energy and use it for other loads or if it is able to store it for later use. For efficiency, however, the current developed by the motor during regeneration must be great enough to provide the necessary field excitation. As is well known, the current generated is a function of motor field current and armature rotational speed. Thus, as the vehicle speed is reduced, the ability of the motor to regenerate energy is also reduced. For a series wound traction motor in which a portion of the armature regenerated current is used as field current for the motor, a point will be reached, at some speed, at which all of the regenerative energy is required just to maintain the field excitation and no energy is available to be retransferred to the power source. At this speed, regenerative braking must be terminated and some form of dynamic braking initiated. In the prior art, two transitions have been undertaken when the vehicle is being braked: a first transition from propulsion to regenerative braking and a second transition from regenerative braking to dynamic braking.
For a series wound electric traction motor, the transition from a propulsion mode to an electrical braking mode requires that current through either the field or armature winding be reversed. The preferred method generally is to reverse the field winding connections. Since this also reverses the polarity of the voltage on the armature (which then acts as a generator), a contactor is normally used to reconnect the armature into the proper configuration for regenerative braking. In many instances the residual flux in the armature and field winding will be sufficient to establish regenerative braking as soon as the reconnection has occurred. In other instances, however, the residual flux may be either of the wrong polarity or may be insufficient to permit regenerative braking to be established. Many prior art systems have dealt with these problems by incorporating a resistance which connects the armature to the power source (usually a battery) in such a manner that a current path through the motor insures initiation of regenerative braking. The problem with this solution is that a resistance, regardless of size, may consume from five to ten percent of the available regenerative energy and create heat dissipation problems.
One method of dealing with these shortcomings is shown in U.S. patent application Ser. No. 288,083, now U.S. Pat. No. 4,423,363 filed July 27, 1981, which application is of common assignee with the present invention. In that method, a controllable resistance is connected between the armature and the power source to provide an initial current path through the armature to establish the proper polarity of flux to enable regenerative braking. While offering a considerable advance over the art, two problems remain even with this method of initiating regenerative braking. First, since most traction motors are not equipped with tachometers, there is no indication of motor speed and a directional change at or near zero speed will result in an attempt to regeneratively brake. A period of time then follows during which there is insufficient torque to produce acceleration or deceleration. The only motor current during this period is that allowed by a resistor inserted for initial field excitation. The second problem is that the use of a resistor initially to establish minimum field current and flux results in an undesirable dissipation of power.
Other problems also arise with prior art controls during the subsequent transition from regenerative braking to dynamic braking. In performing this switching function, field current is normally reduced to zero to avoid producing large current transients. The reduced field current and reduced armature potential permits braking torque to die out and the gearing between the motor and wheels to relax. The subsequent reinitiation of braking torque produces a jerky reaction on the vehicle. Even with the apparatus disclosed in the aforementioned patent application Ser. No. 288,083, now U.S. Pat. No. 4,423,363 there is a short period of time, approximately 50 milliseconds, during which the regulator controlling power to the motor is cut off.
Accordingly, it is among the objects of the present invention to provide an electrical braking control for a direct current traction motor which overcomes such inherent prior art problems as those outlined above.
More particularly, it is an object of the present invention to provide an electrical braking controller which initiates electrical braking in a plug mode of braking and then, when conditions are suitable for regenerative braking, causes a transition to regenerative braking, followed by a return to a plug mode of braking whenever regenerative braking can no longer be efficiently achieved, all of which is carried out smoothly and efficiently without unduly wasting regenerative power.